Compressors for use in an air-conditioning system of a vehicle are rotationally driven by a traction engine to compress a refrigerant. For this purpose, an endless belt running around a follower pulley fixed at the end portion of the rotatable shaft of this compressor and a driven pulley fixed at the end portion of a crank shaft for the above traction engine in order to rotationally drive the above rotatable shaft by the rotation of this endless belt.
FIG. 5 is a view showing the rotational driving structure of a rotatable shaft 1 of a compressor. This rotatable shaft 1 is rotationally supported within a casing 2 by a rolling bearing not shown in the figure. A follower pulley 4 is rotationally supported by a double-row radial ball bearing 5 around a supporting cylinder member 3, which is an instance of the supporting member as recited in a claim, the external surface of the end portion of this casing 2. This follower pulley 4 is designed in the form of a circular ring having a horseshoe cross section within whose inner space is arranged a solenoid 6 which is fixed to the end surface of the above casing 2. On the other hand, an attachment bracket 7 is fixed to the end portion of the above rotatable shaft 1 projected from the above casing 2 while an annular plate 8 made of a magnetic material is supported by a sheet spring 9 around this attachment bracket 7. When the above solenoid 6 is not energized, this annular plate 8 is located separate from the above driven pulley 4 by the elasticity of the above board spring 9 as illustrated in FIG. 5. However, when the above solenoid 6 is energized, this annular plate 8 is attracted and stuck to this follower pulley 4 to allow the transmission of a rotation force from this follower pulley 4 to the above rotatable shaft 1. Namely, the above solenoid 6, the above annular plate 8 and the above sheet spring 9 serve as an electromagnetic clutch 10, in combination, for engaging and disengaging the above follower pulley 4 with the above rotatable shaft 1.
In the case of the above structure in which the double-row radial ball bearing 5 is used to rotationally support the follower pulley 4, even when some unbalanced load is applied to this follower pulley 4 from the endless belt 11 (shown with a phantom line) running around this follower pulley 4, there is little possibility of incurring the misalignment (inclination) between the central axis of the outer race 12 and the central axis of the inner race 13 constituting the above double-row radial ball bearing 5. Accordingly, it is possible to assure the durability of the above double-row radial ball bearing 5 and also to avoid the partial wear of the above endless belt 11 by preventing the center of rotation of the above follower pulley 4 from being inclined.
By the use of the above double-row radial ball bearing 5, however, it is inevitable that the dimension is increased in the axial direction. The rotatable supporting structure of the follower pulley 4 has to be located within a limited space in many cases, and therefore it is undesirable that the dimension is increased in the axial direction. In addition to this, the increased dimension in the axial direction results in the increased cost of the respective constituent parts.
It becomes easy to install the rotatable supporting structure in a limited space by making use of a single-row deep groove type radial ball bearing, in place of the double-row radial ball bearing 5 as described above, as a rolling bearing for supporting the above follower pulley 4. However, in the case of a simple single-row deep groove type radial ball bearing, when a moment load is applied to the above follower pulley 4, there is only a small resisting force to prevent the follower pulley 4 from being inclined to increase the degree of misalignment between the central axis of the outer race and the central axis of the inner race constituting the above double-row radial ball bearing. This results not only in an insufficiency in durability of the above radial ball bearing, but also in a substantial partial wear of the endless belt 11 running around the above follower pulley 4.
Taking into consideration the above circumstances, it is proposed in the prior art to use a single-row four-point contact type radial ball bearing, for example, as described in JP Patent Publication Nos. Tokukai Hei 9-119510 and Tokukai Hei 11-336795. Among them, FIGS. 6 and 7 shows the second example of the conventional structures as described in JP Patent Publication No. Tokukai Hei 9-119510.
In the case of the second example of the conventional structures, a single-row four-point contact type radial ball bearing 14 is used to support, around the supporting structure not shown in the figure, a follower pulley 4a formed by bending process, e.g., by pressing a metallic plate and the like process. This radial ball bearing 14 is provided with an outer race 15 and an inner race 16 which are supported to be concentric with each other, and a plurality of balls 17 and 17. While the inner peripheral surface of the outer race 15 is formed with an outer raceway 18 over the entire circumference, the outer peripheral surface of the inner race 16 is formed with an inner raceway 19 over the entire circumference. Each of the respective outer and inner raceways 18 and 19 has a so-called Gothic arch-like cross section in which arcs having curvature radii larger than a ½ of the diameter of the respective balls 17 and 17 intersect at the center position. Accordingly, each of the respective outer and inner raceways 18 and 19 is in contact with the rolling contact surface of each of the respective balls 17 and 17 at two points, so that there are four contact points in total for each of the respective balls 17 and 17.
The four-point contact type radial ball bearing 14 as described above has a higher rigidity against moment loads as compared with conventional single-row deep groove type radial ball bearings, and therefore even when a moment load is applied thereto the central axis of the above outer race 15 is hardly displaced from the central axis of the above inner race 16. Because of this, it is possible to lessen the partial wear of the endless belt 11 (refer to FIG. 5) rinning around the follower pulley 4 as compared with the case where a rotation support device for a compressor pulley is constructed by the use of a conventional single-row deep groove type radial ball bearing. Meanwhile, the above JP Patent Publication No. Tokukai Hei 11-336795 describes a structure in which a four-point contact type radial ball bearing as described above is installed in the rotatable supporting structure of a follower pulley for driving a compressor while an electromagnetic clutch is further provided between this follower pulley and the rotatable shaft of the compressor.
Also in the case of the single-row three-point contact type ball bearing 14a as illustrated in FIG. 8, the rigidity against moment loads is high as compared with conventional single-row deep groove type radial ball bearings, and therefore even when a moment load is applied thereto the central axis of the above outer race 15 is hardly displaced from the central axis of the above inner race 16a. This three-point contact type ball bearing 14a is provided with an inner raceway 19a having an arcuate cross sectional profile with a single curvature radius on the outer peripheral surface of this inner race 16a, and an outer raceway 18 having a Gothic arch-like cross sectional profile and making in contact with the rolling contact surface of the above ball 17 at two points on the inner peripheral surface of the above outer race 15 in the same manner as the four-point contact type radial ball bearing 14 as illustrated in FIG. 7. In the case of this three-point contact type ball bearing 14a for supporting a compressor pulley, it is also possible to lessen the partial wear of the endless belt 11 (refer to FIG. 5) running around the follower pulley 4 as compared with the case where a rotation support device for a compressor pulley is constructed by the use of a conventional single-row deep groove type radial ball bearing. This applies also to a three-point contact type ball bearing in which the rolling contact surface of each ball is in contact respectively with the outer raceway at one point and with the inner raceway at two points in a reverse manner to the structure of FIG. 8.
As mentioned above, when a three-point contact type or a four-point contact type radial ball bearing is used in the rotatable supporting structure of a follower pulley for driving a compressor, there is a possibility of achieving both the reduction of the size and weight thereof and the durability thereof at a higher level. However, in the case of the three-point contact type or four-point contact type radial ball bearings as described above, there may occur the following shortcomings respectively.
First, in the case of the three-point contact type radial ball bearing among these two types, it is desired to decrease the curvature radius of the cross sectional profile of the raceway being in contact with the rolling contact surface of each ball at one point to 0.505 to 0.520 times the diameter (50.5 to 52.0%) of each ball for the purpose of assuring the rigidity against radial loads. However, if the curvature radius is decreased in this manner, it becomes likely that the respective balls ride over the edge of the above raceways so that an excessive surface pressure due to the edge load may be applied to the rolling contact surfaces of the respective balls. As a result, it becomes likely to cause damage such as premature flaking on the rolling contact surfaces of the respective balls.
On the other hand, in the case of the four-point contact type radial ball bearing, the rigidity against moment loads can be assured but then the rotational resistance and the internal heat generation tend to increase due to the larger number of contact points between the raceways and the respective balls. In addition to this, when a heavy moment load is applied, there is a possibility of displacing the above contact points toward the edge in the width direction. Because of the displacement, a micro slip tends to occur at the respective contact points so that internal heat generation and wear may be increased due to the micro slip.
Incidentally, JP Patent Publication No. Tokukai 2001-208081 describes a single-row deep groove type radial ball bearing having a cross sectional profile of each of the inner raceway and the outer raceway in which arcs having different curvature radii are smoothly joined in the form of a composite arc. However, in the case of the single-row deep groove type radial ball bearing as described in this publication, the rolling contact surface of each ball is in contact with each of the inner raceway and the outer raceway at one point, i.e., as a two-point contact type, and therefore the rigidity against moment loads is considered as smaller than that of the three-point contact type or the four-point contact type as described above.
Taking into consideration the above situation, the present invention has been made in order to provide a rotation support device for a compressor pulley having an excellent durability by assuring the rigidity of a radial ball bearing against moment loads, preventing ball from riding over the end edge of the raceway and preventing the rotational resistance and the internal heat generation from increasing.